1. Field of the Invention
This invention relates to a hydraulic pressure control system for a hydraulically operated vehicle transmission, more particularly to a hydraulic control system for a hydraulically operated vehicle automatic transmission which determines a clutch hydraulic (oil) pressure control value taking into account the clutch engagement state.
2. Description of the Prior Art
In prior-art hydraulic pressure control systems for a hydraulically operated vehicle automatic transmission, it is a common practice to control the hydraulic pressures of the clutches, brakes and other frictional engaging elements. Japanese Laid-Open Patent Application No. Sho 60(1985)-231,056, for instance, teaches a technique controlling the hydraulic pressures applied to frictional engaging elements so as to regulate the rotational speed of a member whose rotational speed changes during gear shifting, such as the transmission input shaft, so as to change following a desired rotational speed change rate.
In the prior-art systems, however, the desired rotational speed change rate is set to be time dependent and does not take the actual engagement state of the frictional engaging elements into consideration. This leads to problems because the viscosity of the oil (automatic transmission fluid (ATF)) varies greatly with temperature, and the engagement state of a frictional engaging element changes with aging. Specifically, cases arise in which the predicted engagement state on which the applied hydraulic pressure control value is predicated differs from the actual engagement state. As a result, unexpected gearshift shock is liable to occur.
An object of this invention is therefore to overcome this problem by providing a hydraulic pressure control system for a hydraulically operated vehicle transmission which avoids the occurrence of unexpected gearshift shock by determining a desired rotational speed change rate taking into account the actual engagement state of the frictional engaging element and by determining the hydraulic pressure control value based thereon.